While some muzzle manufacturers may suggest sizing your dog for a muzzle based purely on its weight and breed, there are enormous variations in snout size and shape between dogs of the same weight and breed. The most effective way of measuring a dog for a muzzle is to check both the length and circumference of its snout. If the dog you're measuring is particularly excitable or aggressive, you may need the help of a friend or even a veterinarian to hold it while you measure.
Measure the length of your dog's snout by placing the "zero" end of the measuring tape between the dog's eyes, even with the inside corner of the eyes. Stretch the measuring tape straight down the middle of your dog's snout and note the measurement at the very end of the dog's nose.
Subtract one half inch from the measurement you just made--this is because most muzzles won't reach all the way up to the dog's eyes but stop just about a half inch short of them. If you have a very large dog, subtract one inch instead of a half inch. The resulting number is the actual length measurement for your dog's snout.
Gauge the circumference of your dog's snout by wrapping the measuring tape around it, one half inch below the inside corners of its eyes. The dog's mouth should be closed as you do this; make sure the tape is snug but not tight or taut.
Write down both measurements and keep them on hand for easy reference when you order your dog's muzzle.